Previous efforts to recycle cellulosic materials, such as waste corrugated materials, boxcuts (i.e., waste similar to cereal boxes or soda cases/cartons), old newspapers, have focused on various methods to treat the cellulosic material before it is reused by itself or mixed with virgin pulp. For example, U.S. Pat. No. 3,440,134 to Murphy discloses a method for producing an acceptable grade of paper utilizing waste corrugated cardboard and corrugated paper. The method of Murphy comprises the steps of comminuting waste corrugated material, forming a water slurry with said waste, adjusting the solids content of the waste in slurry form to approximately 20%, digesting the comminuted waste slurry with caustic soda in an amount consisting of 10% by weight of the waste, heating the slurry with direct steam to approximately 160.degree. C. for about 3 hours, pulping the digested slurry at a temperature of approximately 35.degree. C. for 3 to 5 minutes, increasing the solids content of the resulting pulp to approximately 30%, and bleaching the pulp as required to achieve a desired brightness.
U.S. Pat. No. 5,147,503 to Nguyen discloses a process and an apparatus for recycling waste cellulosic material including corrugated paperboard to produce a cellulosic fiber pulp. The process involves the steps of cooking the waste cellulosic material, for example, corrugated paperboard, in an aqueous alkaline cooking liquor to produce a brown stock pulp having a kappa number lower than that of the waste material. The recycled pulp can be employed as the sole pulp component of brown paper products, or can be bleached to provide a pulp for white products. The brown stock can be mixed with virgin pulp.
As disclosed in the Nguyen patent, paperboard is typically produced from virgin pulp. The production of virgin pulp involves reacting or cooking wood chips with an alkaline cooking liquor at an elevated temperature. Lignin is a component of the wood chips which is dissolved by the cooking liquor in the manufacture of cellulose pulp. The character of the pulp produced is dependent on the extent of lignin removal from the wood chips, and hence on the residual lignin content of the final pulp. The kappa number represents a measure of residual lignin content. Higher kappa numbers indicate higher residual lignin levels. The kappa number of a brown stock pulp obtained from cooking softwood in a kraft liquor is typically 50 to 100, and such a pulp is used for making linerboard of corrugated paperboard. The kappa number of a brown stock pulp obtained from cooking hardwood in a kraft liquor is typically 130 to 160, and such a pulp is used for making the corrugated medium of the corrugated paperboard. The kappa number of a hardwood brown stock pulp would need to be reduced to about 10 to 15, and that of a softwood brown stock pulp to about 25 to 35 to provide a pulp suitable for bleaching to produce white paper products. Corrugated paperboard waste comprising linerboard and corrugating medium has a kappa number of 80 to 120.
U.S. Pat. No. 4,191,610 to Prior discloses the manufacture of corrugated boxes from a corrugated medium consisting of virgin semichemical pulp, corrugated clippings, old corrugated boxes or mixed waste material using waste liquor.
U.S. Pat. No. 3,925,150 to Marsh discloses a method of recycling corrugated paperboard of the type comprising a fluted layer sandwiched between layers of linerboard. The fluted layer is usually made from relatively low grade and short-fibered pulp, such as neutral sulfite semichemical pulp and short-fibered kraft pulp. The liners are commonly made of a good grade of kraft pulp. When corrugated board is pulped for use as waste paper stock, the high-grade kraft constituents of the liners and the relatively lower-grade fiber constituents of the corrugating medium are thoroughly mixed, and the resulting furnish is no longer suitable as liner stock. The Marsh patent discloses a method for separating used corrugated board into two fractions, one being kraft fiber suitable for use as virgin kraft pulp and the other fraction containing the majority of the semichemical pulp and other short-fibered constituents of the corrugated medium.
Canadian Patent No. 1,110,411 discloses a method for recycling waste consisting of paperboard which has been previously impregnated with waxes or resins in a previous converting process, such as wet strength packaging or furniture component manufacture. To avoid the expense of using original kraft cooking liquor (white liquor) to digest the waste paperboard, Canadian Patent No. 1,110,411 proposes to use the effluent by-product of a prior pulping process, i.e., the weak black liquor from the kraft (sulfate) process for making pulp by cooking wood chips under heat and pressure in a solution of sodium hydroxide and sodium sulfide.
None of the aforementioned prior art discloses or suggests to co-pulp waste cellulosic material with wood chips